The present invention relates to a failure diagnostic device for determining a failure of a discharge valve installed in a discharge flow channel of an anode discharge gas of a fuel cell system, and more particularly to improvement technology for increasing the accuracy of failure determination.
In a fuel cell system for generating electric power by supplying a fuel gas and an oxidizing gas to a fuel cell stack comprising a plurality of unit cells stacked in series, a configuration is known in which an unreacted hydrogen gas is reused for a cell reaction by refluxing the hydrogen gas that was discharged from an anode electrode to the anode electrode. A hydrogen discharge valve for discharging part of the hydrogen gas is provided in the circulation flow of the hydrogen gas, and when the concentration of components other than hydrogen in the hydrogen gas becomes high, the hydrogen discharge valve is periodically opened to maintain correctly the concentration of hydrogen supplied to the anode electrode. However, when the hydrogen discharge valve malfunctions and the opening-closing actuation of the valve does not function normally, a failure occurs during cell operation. For this reason, Japanese Patent Publication Laid-open No. 2003-92125 suggested a technology by which a hydrogen discharge instruction is conveyed to a hydrogen discharge valve and the failure of the hydrogen discharge valve is determined based on the target pressure of the fuel supply unit of the fuel cell stack and the actual hydrogen supply pressure. When the hydrogen discharge valve is open, the actual hydrogen supply pressure has to become lower than the target pressure. Therefore, if the difference therebetween is equal to or higher than a threshold, the operation is normal, and if the difference is less than the threshold, a closure failure is determined. Furthermore, when the hydrogen discharge valve is closed, the actual hydrogen supply pressure has to be equal to the target pressure. If the difference therebetween is less than a threshold value, the operation is normal, but if the difference is equal to or higher than the threshold, an opening failure can be determined.